How USB Charging Works, or How to Avoid Blowing Up Your Smartphone

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The tech world has finally coalesced around a charging standard, after years of proprietary adapters and ugly wall-wart power supplies. Well, sort of: We’re already seeing some fragmentation in terms of the new USB-C connector, which could eventually replace USB, as well as what is thankfully turning out to be a short-lived obsession Samsung had with larger USB micro-B connectors for its Galaxy line. But aside from that, and with the obvious exception of Apple’s Lightning connector, micro USB has destroyed the industry’s penchant for custom ports.

Ten to 15 years ago, you always had to make sure you had the correct power supply for each of your gadgets. Usually, that power supply wasn’t even labeled. Today, you can charge your phone at your friend’s house, plug your tablet into any computer, and display photos from a digital camera directly on your TV, all thanks to a standardized connector. In its place, though, there’s a new problem: USB power. Not all USB chargers, connectors, and cables are born equal. You’ve probably noticed that some wall chargers are stronger than others. Sometimes, one USB socket on a laptop is seemingly more powerful than the other. On some desktop PCs, even when they’re turned off, you can charge your smartphone via a USB socket. It turns out there’s a method to all this madness — but first we have to explain how USB power actually works.

New specifications

There are now four USB specifications — USB 1.0, 2.0, 3.0, and 3.1 — in addition to the new USB-C connector. We’ll point out where they significantly differ, but for the most part, we’ll focus on USB 3.0, as it’s the most common. In a USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone, tablet, or camera is the device. Power always flows from the host to the device, although data can flow in both directions, such as when you copy files back and forth between your computer and your phone.

Okay, now the numbers. A regular USB 1.0 or 2.0 socket has four pins, and a USB cable has four wires. The inside pins carry data (D+ and D-), and the outside pins provide a 5-volt power supply. USB 3.0 ports add an additional row of five pins, so USB 3.0-compatible cables have nine wires. In terms of actual current (milliamps or mA), there are three kinds of USB port dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such), and the third kind applies to “dumb” wall chargers.

In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); with USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1,500mA (1.5A). USB 3.1 bumps throughput to 10Gbps in what’s called SuperSpeed+ mode, bringing it roughly equivalent with first-generation Thunderbolt. It also supports power draw of 1.5A and 3A over the 5V bus.

USB-C is a different connector entirely. It’s universal; you can put it in either way and it will work, unlike with USB, and like Apple’s Lightning connector. USB-C is also capable of twice the theoretical throughput of USB 3.0, and can output more power. Apple joined USB-C with USB 3.1 on its 12-inch MacBook and new MacBook Pros. We’re also seeing USB-C on phones in earnest now; current popular models include the Samsung Galaxy S8, the Google Pixel, and the OnePlus 3T. But there can also be older-style USB ports that support the 3.1 standard.

The USB spec also allows for a “sleep-and-charge” port, which is where the USB ports on a powered-down computer remain active. You may have noticed this on your desktop PC, where there’s always some power flowing through the motherboard, but some laptops are also capable of sleep-and-charge.

Now, this is what the spec dictates. But there are plenty of USB chargers that don’t conform to these specs — mostly of the wall-wart variety. Apple’s iPad charger, for example, provides 2.1A at 5V; Amazon’s Kindle Fire charger outputs 1.8A; and many car chargers can output anything from 1A to 2.1A.

Can I blow up my USB device?

There is a huge variance, then, between normal USB ports rated at 500mA, and dedicated charging ports, which range all the way up to 3,000mA. This leads to an important question: If you take a phone which came with a 900mA wall charger, and plug it into a 2,100mA iPad charger, as an example, will it blow up?

In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will explode — and in fact, using a more powerful charger should speed up battery charging. We now do this all the time with our mobile devices here at ExtremeTech, and we’ve never had a problem.

The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. Way back in 2007, the USB Implementers Forum released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your PC’s USB ports, or by using a wall charger. Shortly thereafter, USB devices that implemented this spec started to arrive.

If you have a modern USB device, you should be able to plug into a high-amperage USB port and enjoy faster charging. If you have an older product, however, it probably won’t work with USB ports that employ the Battery Charging Specification. It might only work with old school, original (500mA) USB 1.0 and 2.0 PC ports. In some (much older) cases, USB devices can only be charged by computers with specific drivers installed, but this is now going back more than a decade.

Quick charging and other notes

Many new phones offer some kind of quick-charge capability — often with variable names, the most common being Qualcomm’s Quick Charge that works with Snapdragon-powered phones. The standard evolved quickly, and sits at version 4 at the time of this writing. For quick charging to work, usually you’ll need to use either the power adapter that came with the phone, or an appropriately labeled third-party adapter. Otherwise it will still take several hours to juice up.

There are a few other things to be aware of. While PCs can have two kinds of USB port — standard downstream or charging downstream — OEMs haven’t always labeled them as such. As a result, you might have a device that charges from one port on your laptop, but not from the other. This is a trait of older computers, as there doesn’t seem to be a reason why standard downstream ports would be used, when high-amperage charging ports are available. Most vendors now put a small lightning icon above the proper charging port on laptops, and in some cases, those ports can even stay on when the lid is closed.

In a similar vein, some external devices — 3.5-inch hard drives, most notably — require more power than a typical USB port can provide. That’s why they include a two-USB-port Y-cable, or an external AC power adapter.

Otherwise, USB has certainly made charging our gadgets and peripherals much easier than it ever has been. And with USB-C, things will get even simpler, because you’ll never again have to curse out loud after plugging it in the wrong way.

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